The nozzle must enable circuit breaking to be performed over the entire required interruption window and for currents going up to the short-circuit breaking capacity.
The problem occurs due to the high value of the transient recovery voltage that exists in such very high voltage circuit breakers having only one break per pole.
Many types of insulating blast nozzles are known, but they are not suitable for such applications.
For the gas to be able to recover its dielectric capacity, gas temperatures must be below a critical value of about 2000xc2x0 C., and the density of the blast gas must also be sufficient.
European patent EP 0 028 039 describes a compressed-gas circuit breaker comprising an insulating blast nozzle having an internal profile that comprises a cylindrical portion followed by a conical portion and then by another cylindrical portion.
Experience shows that a nozzle shaped in that way is not optimal since the temperature of the gas is too high in the downstream portion of the nozzle, and its dielectric capacity deteriorates after breaking due to turbulence.
The present invention proposes an insulating nozzle ensuring, in the downstream portion thereof, a better flow of gas and an improved range of temperatures, thus enabling the circuit breaker to withstand a higher voltage after the current has been interrupted.
The invention thus provides an insulating blast nozzle for a circuit breaker, the circuit breaker having a first arcing contact and a second arcing contact that separate from each other when the circuit breaker is opened subsequent to prior separation of permanent contacts, said insulating blast nozzle being secured to one of the permanent contacts and being generally horn-shaped, having a throat and defining a volume of revolution having an upstream portion and a downstream portion joined by the throat, in which the inside volume is cylindrical and closed by the second arcing contact when the circuit breaker is closed and for a few milliseconds after the arcing contacts have separated, the wall of the nozzle in its portion defining the upstream portion of said volume of revolution surrounding said first arcing contact, the downstream portion of said volume comprising a first conical portion, wherein said first conical portion is followed by a second conical portion having a flare angle that is more open than that of said first conical portion.
In practice, the angle of the first conical portion lies in the range 8xc2x0 to 17xc2x0, and is preferably in the range 10xc2x0 to 12xc2x0, and the angle of the second conical portion lies in the range 12xc2x0 to 25xc2x0, and is preferably in the range 14xc2x0 to 17xc2x0.
According to another characteristic and taking account of the separation speed of the arcing contacts, the lengths of said throat, of the first conical portion, and of the second conical portion are dimensioned in such a manner that the end of the second arcing contact is situated inside the first conical portion during an arcing duration lying between a minimum arcing duration and an average arcing duration, and is situated inside the second conical portion during an arcing duration lying between the average arcing duration and a maximum arcing duration.
According to another characteristic, the first conical portion beginning after said cylindrical portion starts with an inside diameter that is greater than the inside of the cylindrical portion, thereby forming a sudden step at the outlet of the cylindrical portion.
This enables an intense blast to be achieved as soon as the second arcing contact leaves said cylindrical portion, referred to as the xe2x80x9cnozzle throatxe2x80x9d, and enables possible duration of arcing to be shorter.